Voltage attenuator



Jan. 23, 1951 w, HEWLETT 2,539,352

VOLTAGE ATTENUATOR Filed June 1946 FIE ZE INVENTOR W/'///'am P. Haw/eff ATTORNEY Patented Jan. 23, 1951 VOLTAGE ATTENUATOR William R. Hewlett, Palo Alto, Calif., assignor, by

mesne assignments, to Hewlett-Packard Company, Palo Alto, Calif., a corporation of California Application June 29, 1946, Serial No. 680,368

2-Claims. 1

This invention relates generally to attenuators such as are employed for developing a predetermined voltage less than a known applied voltage.

In the past voltage attenuators used in connection with signal generators or other high frequency electronic equipment have been primarily of three general types, namely-resistance, mutual inductance, and mutual capacitance. The resistance type of attenuator consistsgenerally of a network of the ladder type with available points of connection to give voltage attenuation in uniform steps. The mutual inductance type of attenuator makes use of input and pickup coils mounted within a conductive shielding tube, and adjustable axially along the tube to vary the mutual inductance. In attenuators of the mutual capacity type two electrostatic condenser electrodes are employed which are adjustable with respect to the spacing or distance between them, to provide a desired degree of attenuation.

The three types of voltage attenuators described above, and other miscellaneousattenuators: which have been available inthe pasahave been subject to certain limitations and disadvantages. to large variations in input and output impedances with frequency, or do'not-afford a relatively high degree of accuracy over a wide frequency range of say from 1 'kilocycle to" 800 megacycles. This latter deficiencyis due toa variety of factors including thepresence of unpredictable'errors which interfere with the a desired accuracy. For example with attenuators of the resistance type errors can be attributed to stray capacity coupling between the various sections of the ladder, the effect of which is particularly noticeable for the higher frequencies It is an object of the present invention to provide a voltage attenuator of the resistance type which will afford a. relativelyhigh degree of accuracy for the step by step attenuation, with retention of this accuracy over a wide frequency range.

A further" object of the invention is to provide a, resistance type of attenuator utilizing a ladder network which will not be subject to unpredictable'err'ors due to stray capacity coupling between the elements of the network and with relatively constant input and output impedances.

Additional objects of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawing.

Referring to the drawing:

Figure 1 is a side elevational view in section In general such attenuators are subject illustrating an attenuator incorporating the present invention.

Figure 2 is a cross sectional view taken along the line 22 of Figure 1.

Figure 3 is a circuit diagram serving to illustrate the ladder network formed by theresistance element of my attenuator.

The attenuator illustrated in the drawing consistsof a hollow conducting tube H3 formed of suitable metal such as brass or copper, and which is preferably circular in cross section. Aligned with and spaced longitudinally along the axis of the tube are the contact elements I 1. These ele ments are preferably in the form of conducting rings made of suitable metal, and provided with annuiarl disposed spring contactor brushes l2, for a purpose to be presently described. In manufacture these brushes can be formed integral witha thin sheet metal spring, annulus [3, which is attached as by soldering to one face of the contactor body. At the input end of the attenuator a cup-like contactor element I4 is provided, and which carries the axial stud conductor 16. The corresponding endof-the tube H3 is provided'with a metal closure 17, which has a-central aperture l8" to receive-one-endor-a coaxial transmission line. The central conductor of the transmission line can make direct connection with the stud conductor l6.

A plurality of conducting rings t9 surround the contact elements H and are dimensioned to loosely fit within the tube I0. Split metal spacer rings 2| are interposed 'betweenthe rings i9, and are normally sprungoutwardlyto have frictional and'electrical contact with'the' tube 10. They serve to insure "grounding of rings 19 to tube i0 and to maintain accurate spacing along the length of the device.

A plurality of resistors 23 of proper value connect between the contact elements H and the conductingrings-ifi. Preferably these resistors are distributed symmetrically about the axis of the attenuator, and for example three such resistors displaced apart can be used with each contact element'l l. nect'between the successive contactor elements ll. These resistors are likewise symmetrically distributed and assuming that three such resistors are employed as in the case of the resistors 23, the distribution. can be as illustrated in Figure 2. ,It is convenient to utilize resistors having a fair degree of mechanical strength with relatively rigid extending terminal wires. Soldering of such terminal wires to the contactor elements I I and the outer rings for the resistors 23, and to Additional resistors 26 con t: the successive contactor elements for the resistors 2t, affords a structure of considerable mechanical strength sufficient to retain the contactor elements i i in proper position along the axis of the device.

The output end of the attenuator is provided with a metal closure 23 having a central opening 29. The inner face of this closure is recessed to seat the last one of the series of contactor elements H as illustrated. Opening 29 serves to accommodate a probe M which can be inserted into the attenuator for the purpose of making electrical connection with a selected one of the contactor elements i l. The probe illustrated consists of an outer conductor sleeve 32, within which is an axial conductor 33. At the inner end of the probe the sleeve 32 is provided with an insulator 3 3, through which the adjacent end of the conductor 33 extends. The projecting end of the conductor carries a contacting element 35 in the form of an enlarged metal head which is circular in transverse cross section, and which is dimensioned for good electrical connection with the resilient brushes it? of the contactor elements H.

The probe is of sufiicient length whereby the contactor element 3% can be inserted to make engagement with any one of the contactor elements I l or the input contactor element it. The inner end of the sleeve 32 is also proportioned to make electrical engagement with all of the contactor elements it on the output side of that element which is connected to the central conductor of the probe. The outer end of the probe is coupled to a suitable coaxial transmission line 3?! which has its outer hollow conductor grounded, whereby the sleeve 32 of the probe and the tube iii of the attenuator are likewise grounded. Thus sections of the ladder network which are on the output side of the last section being contacted by the probe are grounded out, thus avoiding possible intermediate errors.

The ladder type network formed by the construction described above is illustrated diagrammatically inFigure 3. The resistors 23 are considered as grouped to form the resistance elements R1,; and the resistors 28 are grouped to form the resistance elements R2. Conductor illa is representative of the tube it, which is grounded together with the sleeve 3.2a (representing the probe sleeve 32).

The attenuator can also be constructed of resistors having uniformly distributed constants such as disc resistors and resistive tubes. By proper choic of dimensions, it is possible to construct this attenuator of uniformly distributed resistors so that for all elements, both series and shunt P/t=a constant where p=the restivity of the resistance material t=thickness of resistance material.

It would thus be possible to construct this attenuator by initially assembling the supports for the resistors only, and then coating these supports uniformly to the required thickness with some resistive material.

A feature of my attenuator is that the efiective inner diameter of the tube it (i. e., the inner is made substantially greater than the step by step attenuation of the ladder network. This makes it possible to reduce errors due to stray capacity coupling to relatively low and insignificant values. I have found that insofar as capacity coupling between the ladder sections is concerned, the cylindrical tube iii acts as a wave guide below cut-off frequency. Therefore the attenuation of stray capacity coupling can be calculated on the basis of known wave guide theory. Thus per inch lineal distance along the length of the tube the stray capacity coupling sufiers an attenuation in db. (decibels) which can be expressed by an equation as follows:

where diameter of split rings 2 i) and the length between a TMci mode. is the wave length in inches of the signal applied.

Assuming that 7\ )\c then Since the mode Tlvlo1 furnishes the unwanted coupling where a is the tube it radius in inches therefore a 8.68 X db./in.

Assuming that the ladder network is such that each section has of itself an attenuation of (la,

then the length of each section, Z, should be such that Assuming that Z is the desired output impedance of the attenuator, Z0 is the necessary source impedance of the generator feeding the attenuator, and 'y is the reduction of voltage per section,

then

In practice it is desirable to construct the attenuator in such a manner that the attenuation is in uniform steps, for example 10 db. for each section. The coaxial probe should be designedso that its characteristic impedance is the same as the desired output impedance.

One attenuator which has been constructed in V accordance with the foregoing used groups of three resistors is illustrated in the drawing. The attenuation for each step was 10 db. and the 11' output impedance was 50 ohms. For the frequencies of 1 kc., 3O mc., 300 mc., 500 mo. and 800 mc., the attenuation was substantially that'cal culated. Looking back into the output the voltage standing wave ratio was less than 1.2 for all frequencies below 800 Me. when measured with a generator having a source of impedance of 50 ohms.

It will be evident that the invention canrbe modified in various ways within the scope of the invention. The number of resistors forming each element of the network can be varied, or if desired resistors having uniformly distributed constants can be used, such as disc resistors and reslstive tubes.

I claim:

1. In a voltage attenuator for use with high frequency electrical equipment, a hollow conducting tube, a plurality of conducting rings disposed axially within the tube and spaced longitudinally of the same, a plurality of resistors disposed within the tube and connected to form with said tube an attenuating network of the ladder type, said network including, resistors connected between'adjacent conducting rings and resistors connecting between the resistor rings and the conducting tube, said network providing step-by-step voltage attenuation for selective connections to said conductor rings, means for applying a voltage input to one end of the network at one end of the tube, and means including a probe adapted to be slidably inserted into the other end of the tube and through said conducting rings for making selective connection with a selected one of said rings to thereby provide an attenuated output.

2. In a voltage attenuator for use with high frequency electrical equipment, a hollow conducting tube, a plurality of conducting rings disposed axially within the tube and spaced longitudinally of the same, a plurality of resistors disposed within the tube, certain of said resistors being connected between adjacent rings and spaced circumferentially with respect to each other, other of said resistors being spaced circumferentially and extending radially from each ring to the adjacent portion of the surrounding tube, said resistors together with said tube and said conducting rings forming a resistive attenuating-network of the ladder type providing step-by-step voltage attenuation for selective connections to said conductor rings, means for applying a voltage input to one end of the network at one end of the tube, and means forming a coaxial probe slidably insorted in the other end of the tube, said probe comprising a shield tube and an inner conductor disposed axially of the shield tube and provided with an enlarged metal head disposed beyond the end of the shield tube, said head serving to make electrical contact with a selected conductor ring, said shield tube serving to contact and ground the remainder of the conductor rings intervening between the selected ring and the output end of the first mentioned tube.

WILLIAM R. HEWLETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 722,453 Murphy Aug. 25, 1902 1,081,843 Larson Dec. 16, 1913 1,621,233 Campbell Jan. 7, 1927 1,957,538 'Jensen June 13, 1931 2,151,157 Schelkunofi Oct. 31, 1936 

